1. Field of the Invention
This invention relates to a method for preparing a silicone rubber which exhibits a minimal difference in physical properties including hardness, tensile strength, elongation, restitution, and compression set between primary vulcanization and secondary vulcanization so that secondary vulcanization may be omitted.
2. Prior Art
In general, silicone rubber is improved in weather resistance, durability, heat resistance, physiological inertness, coloring, etc. and thus used in a wide variety of fields including building materials, electric and electronic parts, business machine parts, automobile parts, and medical equipment parts. Molded silicone rubber parts for use in these applications are often produced by various molding procedures including compression molding, extrusion molding and injection molding during which they are subject to primary vulcanization. Secondary vulcanization is then effected for stabilizing physical properties. In many cases, physical properties such as hardness, tensile strength, elongation, resilience, and compression set largely change between primary vulcanization and secondary vulcanization. Such changes are undesirable from the standpoint of stabilizing the properties of molded parts.
Prior art methods for producing silicone rubber experiencing a little change of physical properties between primary vulcanization and secondary vulcanization are by increasing the content of an alkenyl group serving as a crosslinking site in the polymer, by blending an alkenyl group-containing silane or siloxane, by effecting heat treatment in the presence of a minor amount of a silanol group condensing catalyst, and by surface modifying a reinforcing filler.
These methods, however, have several drawbacks. For example, when the content of an alkenyl group serving as a crosslinking site in the polymer is increased or when an alkenyl group-containing silane or siloxane is blended, the crosslinked silicone rubber would have a significantly increased hardness or reduced elongation. When heat treatment is effected in the presence of a minor amount of a silanol group condensing catalyst, the residual catalyst can detract from heat resistance and exacerbate the processability of unvulcanized rubber compound. The surface modification of a reinforcing filler is effective to some extent, but not to a fully satisfactory extent and adds to the cost of manufacture.